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flippa.com - is good when you have actually developed something on the domain
sedo.com - good for parking domains and setting prices. They will handle transfers also.
bido.com - people have to vote on which ones make it into the auctions I think
digitalpoint forums - I think they have a marketplace.

Domains are not easy to sell and can take years before someone is interested. I've had hundreds of domains, and I have only sold one. That was on Sedo. I've let many go, and HugeDomains.com keeps buying them. It is funny I let a domain expire that I paid around 10 for, and then the next day it is 1400 on HugeDomains.com. I think they are domain squatters.

Never sold any, but let several expire.. I used to have many great ideas for new sites, but finally it ended up with a one page setup if anything at all.. I lacked the man power or hard working quality.. But I still think there was some good ideas of new sites, simply I didn't do them, that's it.

Sedo.com is ok. They have an auction for domains. I mostly use sedo.com as a broker. That's where sedo excels. By broker I mean. When u have someone wanting a domain you have. Go to sedo and make an account. It is better to make one before you have a buyer and get familiar with sedo. But once you have a buyer and an account at sedo then go into your account and setup a broker deal. You can broker it yourself or you can email a sedo broker to be the go between. They will put your domain in escrow and then get the funds from the buyer and put the money in escrow until it is confirmed. Once it is confirmed they will release control of the domain and at the same time they transfer the funds to you. They do charge 10% of the selling fee to the seller. What I normally do is tell the buyer that I am back charging that broker fee and most don't mind.
I have sold roughly $122.000 in domains this year.
Build a single page webpage with adsence or something and park it on a host.
Not pushing this here but what I have done is, I have some clients that I build sites for. Part of my deal is that I will discount what I charge them a little but they sign off that I get to have ad space to sell either ads or link backs. With me it has worked well because one client gets upwards of 100.000 unique visitors a month. Some of the others will get from 20.000 to 60.000 per month. I may charge someone as little as $10.00 per month to place a link back to charging $50.00 per month to put a link to a business site to charging $100.00 a month for a graphic ad.

AfterNic is known as one of the better domain pushers. I use their premium service because there is better traffic, or so they say. You can also park your domains there and make some small change cash while you wait for it to sell. (I've got $20 or so bucks sitting in my account from parking.)

As far as getting a new domain I use 'Moniker'. They're recognized by quite a few of the domain auction sites if you want to use a premium service.

I have kind of got addicted to buying domains over the past few years and now have about 50.
What is the best way to try and sell some, are there auction sites or similar?

Before selling any domain I advice you to appraise the domain to collect its market value.It will give you a rough idea about the range of the domain and hence will serve as a guidance in fixing the selling price.You need not spend to appraise your domain.I found a free domain appraisal service from Thewebpole.com ,use it to appraise all your domains and quote the price accordingly.You can use Flippa,Sedo to sell the domain.
All the best for your sale!!!

If you put your site up for auction/sale make sure that your claims for income are valid and
can be verified. I think 'Flippa' has some rules covering that. I's ok to say that you haven't had any income but it is bad form to make claims that you can't prove.